1. Field of the Invention
The present invention relates to antifreeze, and particularly to antifreeze used as coolant.
2. Description of the Related Art
The internal combustion engine which is applied to automobiles and so on is cooled by the coolant. As the coolant, antifreeze is used in order to prevent freezing up in the cold season. As the antifreeze, generally, the one which is diluted with water after adding rust inhibitor to glycols which is used as freezing-point depressant is used. Especially, ethylene glycol is often used as such freezing-point depressant. Propylene glycol which gives less environmental load than ethylene glycol is desired to be added in the antifreeze as the freezing-point depressant.
Aluminum, cast iron, steel, brass, solder, copper, and so on are existing in the cooling system of the internal combustion engine. Accordingly, a corrosion-preventing effect on the above materials is required for the coolant, and the coolant contains various corrosion inhibitors. Especially, the corrosion-preventing effect on aluminum is required, because the amount of aluminum used in the coolant system tends to increase for the purpose of saving the weight of automobiles. Therefore, the antifreeze which includes propylene glycol having less environmental load is desired as the freezing-point depressant and having much corrosion-preventing effect on the metal is desired.
The antifreeze which costs low and has a superior corrosion-preventing effect is disclosed in Japanese Laid Open Patent Application (JP-P-Heisei 8-85782). The propylene glycol type coolant which is not diluted with water is disclosed in International Publication (WO95/07323). The antifreeze whose corrosion-preventing effect is improved is disclosed in International Publication (WO01/002511). The antifreeze whose corrosion-preventing effect on aluminum is remarkably improved is disclosed in Japanese Laid Open Patent Application (JP-P-Heisei01-315481). The coolant composition which is especially effective for automobiles and so on which tends to include aluminum alloy in the engine coolant is disclosed in Japanese Laid Open Patent Application (JP-P-Heisei04-59885).